The onion plant is believed to originate from West or Central Asia. In Europe it has been known since the bronze ages. The bulbs of the onion plants,—the “onions”— are used in many dishes and have a very healthy reputation. Plant breeding has been focused on yield, appearance, harvestability, storability, flavor and content as onions contain several compounds that have beneficial effects on health. Some of these compounds are most effective when the onion is consumed fresh and their concentrations are often linked with the solids level of onions. A high solids onion that is mild and sweet enough to be consumed without cooking will deliver more health promoting compounds in the diet.
Onion varieties are characterized by day length; “long-day” onion varieties will stop forming tops and begin to form bulbs when the day length reaches 14 to 16 hours while “short-day” onions will start making bulbs in early spring or in autumn/winter when there are only 10 to 12 hours of daylight. “Long-day” onions are usually produced in northern countries or northern states of the USA (north of the 36th parallel; especially latitude) 38°-48° while “short-day” onions are produced in countries or states south of that line (especially below latitude 32°). Long-day onion varieties generally have a more pungent flavor than short-day varieties, which are sweet. Long-day varieties also store better and longer than short-day varieties because they have a relatively higher dry matter content or higher percentage of soluble solids (SSC) compared to short-day onions (see e.g., “Onion Planting” publication, obtainable from the Texas A&M University horticulture website The long storage ability of long-day onion varieties provides the possibility to market onions during late summer, fall and winter (August-March/April) when mild, short-day onions are not available or scarce. Long-day onions are bi-annual for seed production. Seeding for seed production purposes occurs in autumn, possibly, but not necessarily, followed by transplanting in spring. Seed is harvested the next summer. For bulb production long-day onions are seeded early spring, harvested in autumn and subsequently stored over winter. Short-day onions can be seeded in autumn and harvested in spring the next year, or seeded in spring and harvested in early summer of the same year. As the storage ability of short-day onions is low, the availability of these mild onions is restricted to spring-early summer (April-July).
Pungency is the typical onion flavor or taste, caused by the conversion of sulphur containing flavor precursors—alk(en)yl-L-cysteine-sulfoxides (ACSOs)—by the enzyme allinase into thiosulfonates when the onion cells are cut or damaged. A by-product of this enzymatic process, pyruvate or pyruvatic acid is measured as an indicator of the pungency (Schwimmer and Weston 1961, J. of Agric. Food Chem. 9: 301-4). The amount of pyruvate produced is directly related to onion pungency as determined by taste panels (Schwimmer and Guadagni, 1962, J. Food Sc. 27:94-97).
Pungency is an important commercial trait as consumers favour fresh onions with low pungency and sweet taste. Pungency masks the sweet taste of the sugars, which are present in the onion as part of the water-soluble solids or carbohydrates. Pungency is strongly influenced by the presence or absence of sulphur in the soil or plant nutrients (Randle 1992, Euphytica 59: 151-156 and Randle and Bussard 1993, J. Amer. Soc. Hort. Sci. 118: 766-770), but has also a clear genetic component as shown by Lin (1995, J. Americ. Soc. Hort. Sci. 120: 119-122), Simon (1995, Euphytica 82: 1-8), Wall et al. (1996, Euphytica 87: 133-139) and Wall and Corgan (1999, Euphytica 106: 7-13). Pungency can, therefore, vary between locations and between years.
Dry matter in onions consists of both soluble and insoluble carbohydrates. The soluble solids are in the form of fructose, sucrose, glucose, fructans and other saccharides. The analysis of dry matter can be time consuming and destructive for the bulbs. Several researchers have determined that dry matter content and refractive index (soluble solids content) are positively correlated with the percentage of dry matter and the refractive index determination avoids destruction of the bulbs (Mann and Hoyle, 1945, Proc. Americ. Soc. Hort Sci. 46: 285-292; Foskett and Peterson, 1949, Proc. Americ. Soc. Hort Sci. 55: 314-318). Low pungency in onions is strongly correlated with low dry matter content or a low percentage of soluble solids (see further below). Short-day onions, thus, have a low pungency and a low SSC at harvest, and cannot be stored for long periods. For the fresh onion market in northern countries or northern states of the USA (i.e., for long-day countries), however, there is a long existing need for low pungency varieties. This requires long-day onions that combine the properties ‘low pungency’ with ‘high solids’. Such onions do not yet exist in the art, because there is an alleged genetic linkage between the properties ‘high pungency’ and ‘high (soluble) solids’. Thus, long-day onions have a high pungency and a high SSC, whereby they can be stored throughout the winter.
This linkage between high pungency and high SSC is, for example, illustrated by a study of Galmarini et al. (2001, Mol. Gent. Genomics 265: 543-551) wherein molecular markers which were significant for pungency were also significant for SSC, suggesting that this characteristic may be controlled by the same chromosome region. It implies a genetic linkage or association between these traits, resulting in short-day onions, which generally have a low soluble solid content together with a low pungency and long-day onions having a high soluble solid content combined with high pungency.
Also other studies support the strong linkage between the two traits—SSC and pungency (Schwimmer and Weston, 1961, supra; Randle 1992, supra; Simon 1995 supra; Lin 1995; MacCallum et al. 2001, NZ J. of Crop and Hort. Sci. 29: 149-158; Galmarini 2001, supra). For example Simon (1995, supra) observes a strong correlation between pungency and SSC in the parent lines, the F1, F2 and BC1 generations of a diallel between 4 parent inbred lines. Galmarini et al. (2001, supra) and Havey et al. (2004, Genome 47: 463-468) found a phenotypic and genetic significant positive correlation between solids and pungency in the F3 generation.
Galmarini et al. and Havey et al. suggest that this linkage may be the result of pleiotropic effects. There is physiological evidence for this scenario as the higher accumulation of fructans in high solids onions, because of no hydrolization of fructans to fructose and less water uptake, is associated with greater thiosulfinate concentrations, yielding strong correlations among soluble carbohydrates, pungency and onion-induced in vitro anti platelet activity (OIAA). The increase in water content and free fructose in low solids onions could be responsible for diluting the compounds related to pungency and increase the sweeter and milder taste. The QTL analysis as discussed in these articles shows a strong linkage in one group (E) between dry matter percentage (DM %), pungency and OIAA, while DM % and solids are strongly linked in a different group (D). This implies a strong association between DM %, soluble solids, pungency and OIAA, which would be difficult to overcome.
According to some reports (Shock et al. 2004: “Pungency of Selected Onion Varieties Before and After Storage”, Oregon State University, Malheur Experiment Station Special Report 1055: 45-46) pungency may significantly increase during storage. There is, therefore, a need for onions which have a low pungency and high SSC at harvest and whereby the pungency does not increase significantly during storage. In particular, there is a need for onions which have a low pungency and high SSC at harvest, and a low pungency and high SSC after at least about 2, 3, 4, 5, 6, 7, 8 or more months of storage. There is especially a need for low pungency long day onions whereby the pungency does not increase during storage but remains constant or decreases during storage (compared to the level at harvest), i.e. is lower after at least about 2, 3, 4, 5, 6, 7, 8 or more months of storage compared to the level at harvest. A “decrease during storage” refers, thus, to the level after a specific period of storage (e.g. after about 2, 3, 4, 5, 6, 7, 8 or more months of storage) is lower than at harvest. For example if the mean PAD measurement at harvest is 4.92 and after about 3 months is 4.62, then the level has decreased during the 3 months storage period.